Metal re-apply: throw an NSException when attempting to draw with inv…

…alid program (google#7741)

filament/backend/src/metal/MetalDriver.mm

@@ -1634,7 +1634,7 @@
         return;
     }
 
-    ASSERT_PRECONDITION(bool(functions), "Attempting to bind an invalid Metal program.");
+    functions.validate();
 
     auto [fragment, vertex] = functions.getRasterFunctions();